Are federations worth it over vassalization?

I was browsing around for a new empire to play and happened upon an empire that uses brain poacher and genesis arks to make 1 planet vassals.

After that forming a federation with vassals etc to use them to make your research via brain poacher. In the post the OP had talked about getting 1k science / month but this was before the rework.

So my question is as the title states - is there a point to federations? Why wouldn't I just vassalize everything? I'm not NEW to stellaris by any means but this is an aspect I havent ever played with.